Structure and thermal stability of Fe : Al2O3 nanocomposite films

Abstract

Nanocomposite films consisting of Fe nanocrystals (NCs) embedded in an amorphous Al2O3 matrix have been fabricated by alternate pulsed laser deposition in vacuum. The size, shape and distribution of the Fe NCs have been analysed by high resolution electron microscopy. The thermal stability of the films has been studied by in situ annealing in the electron microscope, and the chemical state of the Fe has been investigated by electron energy loss spectroscopy. The nucleation and growth of the Fe NCs has been studied as a function of the number of pulses. Nucleation is homogeneous over the surface of the Al2O3 at the beginning of deposition. The distribution of the NCs is uniform and their in-plane shape can be approximated by ellipses for a small number of pulses on the Fe target. As the Fe content increases some adjacent NCs coalesce resulting in a sudden increase in dimensions and in-plane aspect ratio. Isothermal annealing treatments for up to 160 min show that the Fe NCs are stable up to 400°C, with morphological changes occurring at higher temperatures. The as-prepared Fe NCs were found to be α-Fe and no changes of oxidation state of Fe were observed after annealing.